Preheat the oven to 180°C, fan 160°C, gas 4. Grease a 20cm round springform tin and line with baking parchment.
To make the shortbread base, put the flour, sugar and butter in a food processor and whizz until it resembles breadcrumbs. Pat together to form a ball, then press the dough evenly into the bottom of the tin. Prick all over with a fork, then bake for about 18-20 minutes, until just dry to the touch. Allow to cool completely, which should take about 30 minutes.
To make the mousse topping, put the chocolate and butter in a bowl and melt over a pan of simmering water, making sure that the bowl doesn't touch the water.
Tip the egg whites into a large bowl and whisk until stiff peaks form. In another bowl, whisk the egg yolks with the sugar until pale. Add the chocolate mixture to the egg yolks, then carefully fold in the egg whites until all the mixture is combined.
Pour into the cake tin and bake for 15-20 minutes. Allow to cool, then dust with cocoa powder before serving.
Cook's tip: instead of making a shortbread base, make a biscuit one. Simply whizz some digestive biscuits with some melted butter, then chill before adding the mousse topping.